As a conventional method for power supply to an IH coil in a fixing device using the IH coil, a so-called on-off control system is employed in which the surface temperature of a fixing belt is detected, and if a target temperature is not reached, the maximum power is supplied from a heating source, and after the target temperature is reached, the supply from the heating source is reduced or turned off. A conventional image forming apparatus has two operation modes, that is, a normal paper mode for forming an image on a normal paper having a relatively small basis weight of sheet, and a thick paper mode for forming an image on a thick paper having a large basis weight of sheet. In the normal paper mode, the carrying speed of the fixing belt in the fixing device is a normal speed. On the other hand, in the thick paper mode, deceleration running is carried out, for example, at a ⅓ speed of the normal speed in order to sufficiently fix an image to the thick paper having a large basis weight.
However, in the thick paper mode, because of the large basis weight of sheet, even if the fixing belt is caused to run at a decelerated speed, the surface temperature of the fixing belt does not quickly reach a target temperature particularly when a fixing member is cooled. Therefore, maximum power is applied. Consequently, there is a problem of increased temperature ripple. This temperature ripple is a phenomenon that the surface temperature of the fixing belt changes above and below a target temperature in a vibrating manner. It is considered that this is due to an excessive quantity of heat given to the fixing belt by the fixing device.
Moreover, a recent environmentally friendly fixing device has a fixing component with less heat capacity in order to reduce warm-up time. If such a member is used, the temperature ripple tends to be more conspicuous as large power is supplied to the fixing device. Particularly, if a so-called divided IH coil heating system is employed which uses different coils as IH coils at the center and both sides in the direction of width of the fixing belt, the temperature ripples increase further. This is because a large increase in belt temperature tends to cause a temperature difference between the center coil and the side coil which are alternately driven, and therefore the duty factor of driving pulses is increased. For power supply in feedback for once, the same quantity of power is supplied to the center coil and the side coil. Therefore, a vicious cycle occurs that the large duty factor causes increase in temperature difference. This causes uneven gloss, and in the worst case, it causes high-temperature offset. Moreover, because of the rising temperature within the machine, reduction in life of electronic components arranged near the fixing unit and fixation of toner thereto tend to occur.
In the conventional fixing device, temperature on the fixing belt is detected by a thermopile. The cycle of giving feedback in accordance with the temperature as a result of detection is the same cycle (200 ms) for both the normal paper mode and the thick paper mode. If the duty time is changed in accordance with the temperature difference between the center coil and the side coil but the temperature difference is not resolved in a prescribed time period, the maximum power is supplied to both coils. Therefore, in the thick paper mode, since the carrying speed is slow, the same feedback cycle as in the normal paper mode causes the maximum power to be supplied immediately and therefore a temperature ripple tends to occur.
It is an object of the invention to provide an image forming apparatus having a fixing device in which the conventional problems are improved.